Internet Engineering Task Force                                 T. Lemon
Internet-Draft                                               S. Cheshire
Intended status: Standards Track                              Apple Inc.
Expires: 22 May 21 June 2021                                   18 November December 2020

     Service Registration Protocol for DNS-Based Service Discovery
                        draft-ietf-dnssd-srp-06
                        draft-ietf-dnssd-srp-07

Abstract

   The Service Registration Protocol for DNS-Based Service Discovery
   uses the standard DNS Update mechanism to enable DNS-Based Service
   Discovery using only unicast packets.  This makes it possible to
   deploy DNS Service Discovery without multicast, which greatly
   improves scalability and improves performance on networks where
   multicast service is not an optimal choice, particularly 802.11
   (Wi-Fi) and 802.15.4 (IoT) networks.  DNS-SD Service registration
   uses public keys and SIG(0) to allow services to defend their
   registrations against attack.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 22 May 21 June 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Service Registration Protocol . . . . . . . . . . . . . . . .   5
     2.1.  Protocol Variants . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Full-featured Hosts . . . . . . . . . . . . . . . . .   5
       2.1.2.  Constrained Hosts . . . . . . . . . . . . . . . . . .   6
       2.1.3.  Why two variants? . . . . . . . . . . . . . . . . . .   6
     2.2.  Protocol Details  . . . . . . . . . . . . . . . . . . . .   6
       2.2.1.  What to publish . . . . . . . . . . . . . . . . . . .   7
       2.2.2.  Where to publish it . . . . . . . . . . . . . . . . .   7
       2.2.3.  How to publish it . . . . . . . . . . . . . . . . . .   8
       2.2.4.  How to secure it  . . . . . . . . . . . . . . . . . .   9
       2.2.5.  Service Behavior  . . . . . . . . . . . . . . . . . .   9
     2.3.  SRP Server Behavior . . . . . . . . . . . . . . . . . . .  11  12
       2.3.1.  Validation of Adds  . . . . . . . . . . . . . . . . .  11  12
       2.3.2.  Valid SRP Update Requirements . . . . . . . . . . . .  13  14
       2.3.3.  FCFS Name And Signature Validation  . . . . . . . . .  14
       2.3.4.  SRP Update response . . . . . . . . . . . . . . . . .  14  15
       2.3.5.  Optional Behavior . . . . . . . . . . . . . . . . . .  14  15
   3.  TTL Consistency . . . . . . . . . . . . . . . . . . . . . . .  15  16
   4.  Maintenance . . . . . . . . . . . . . . . . . . . . . . . . .  16  17
     4.1.  Cleaning up stale data  . . . . . . . . . . . . . . . . .  16  17
   5.  Sleep Proxy . . . . . . . . . . . . . . . . . . . . . . . . .  17  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18  19
     6.1.  Source Validation . . . . . . . . . . . . . . . . . . . .  18  20
     6.2.  SRP Server Authentication . . . . . . . . . . . . . . . .  19  20
     6.3.  Required Signature Algorithm  . . . . . . . . . . . . . .  19  21
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  19  21
   8.  Delegation of 'service.arpa.' . . . . . . . . . . . . . . . .  20  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20  21
     9.1.  Registration and Delegation of 'service.arpa' as a
           Special-Use Domain Name . . . . . . . . . . . . . . . . .  20  21
     9.2.  'dnssd-srp' Service Name  . . . . . . . . . . . . . . . .  20  22
     9.3.  'dnssd-srp-tls' Service Name  . . . . . . . . . . . . . .  20  22
     9.4.  Anycast Address . . . . . . . . . . . . . . . . . . . . .  21  22
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21  23
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  22  23
   12. Informative References  . . . . . . . . . . . . . . . . . . .  23  25
   Appendix A.  Testing using standard RFC2136-compliant servers . .  24  26
   Appendix B.  How to allow services to update standard
           RFC2136-compliant servers . . . . . . . . . . . . . . . .  25  27
   Appendix C.  Sample BIND9 configuration for
           default.service.arpa. . . . . . . . . . . . . . . . . . .  25  27

   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26  28

1.  Introduction

   DNS-Based Service Discovery [RFC6763] is a component of Zero
   Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap].

   This document describes an enhancement to DNS-Based Service Discovery
   [RFC6763] that allows services to register their services using the
   DNS protocol rather than using Multicast DNS [RFC6762] (mDNS).  There
   is already a large installed base of DNS-SD clients that can discover
   services using the DNS protocol.

   This document is intended for three audiences: implementors of
   software that provides services that should be advertised using
   DNS-SD, implementors of DNS servers that will be used in contexts
   where DNS-SD registration is needed, and administrators of networks
   where DNS-SD service is required.  The document is intended to
   provide sufficient information to allow interoperable implementation
   of the registration protocol.

   DNS-Based Service Discovery (DNS-SD) allows services to advertise the
   fact that they provide service, and to provide the information
   required to access that service.  DNS-SD clients can then discover
   the set of services of a particular type that are available.  They
   can then select a service from among those that are available and
   obtain the information required to use it.  Although DNS-SD using the
   DNS protocol (as opposed to mDNS) can be more efficient and
   versatile, it is not common in practice, because of the difficulties
   associated with updating authoritative DNS services with service
   information.

   Existing practice for updating DNS zones is to either manually enter
   new data, or else use DNS Update [RFC2136].  Unfortunately DNS Update
   requires either that the authoritative DNS server automatically trust
   updates, or else that the DNS Update client have some kind of shared
   secret or public key that is known to the DNS server and can be used
   to authenticate the update.  Furthermore, DNS Update can be a fairly
   chatty process, requiring multiple round trips with different
   conditional predicates to complete the update process.

   The SRP protocol adds a set of default heuristics for processing DNS
   updates that eliminates the need for DNS update conditional
   predicates: instead, the SRP server has a set of default predicates
   that are applied to the update, and the update either succeeds
   entirely, or fails in a way that allows the registering service to
   know what went wrong and construct a new update.

   SRP also adds a feature called First-Come, First-Served Naming, which
   allows the registering service to claim a name that is not yet in
   use, and, using SIG(0) [RFC2931], to authenticate both the initial
   claim and subsequent updates.  This prevents name conflicts, since a
   second SRP service attempting to claim the same name will not possess
   the SIG(0) key used by the first service to claim it, and so its
   claim will be rejected and the second service will have to choose a
   new name.

   Finally, SRP adds the concept of a 'lease,' similar to leases in
   Dynamic Host Configuration Protocol [RFC8415].  The SRP registration
   itself has a lease which may be on the order of an hour; if the
   registering service does not renew the lease before it has elapsed,
   the registration is removed.  The claim on the name can have a longer
   lease, so that another service cannot claim the name, even though the
   registration has expired.

   The Service Registration Protocol for DNS-SD (SRP), described in this
   document, provides a reasonably secure mechanism for publishing this
   information.  Once published, these services can be readily
   discovered by DNS-SD clients using standard DNS lookups.

   The DNS-SD specification [RFC6763], Section 10 ("Populating the DNS
   with Information"), briefly discusses ways that services can publish
   their information in the DNS namespace.  In the case of mDNS, it
   allows services to publish their information on the local link, using
   names in the ".local" namespace, which makes their services directly
   discoverable by peers attached to that same local link.

   RFC6763 also allows clients to discover services using the DNS
   protocol [RFC1035].  This can be done by having a system
   administrator manually configure service information in the DNS, but
   manually populating DNS authoritative server databases is costly and
   potentially error-prone, and requires a knowledgable network
   administrator.  Consequently, although all DNS-SD client
   implementations of which we are aware support DNS-SD using DNS
   queries, in practice it is used much less frequently than mDNS.

   The Discovery Proxy [RFC8766] provides one way to automatically
   populate the DNS namespace, but is only appropriate on networks where
   services are easily advertised using mDNS.  This document describes a
   solution more suitable for networks where multicast is inefficient,
   or where sleepy devices are common, by supporting both offering of
   services, and discovery of services, using unicast.

2.  Service Registration Protocol

   Services that implement SRP use DNS Update [RFC2136] [RFC3007] to
   publish service information in the DNS.  Two variants exist, one for
   full-featured hosts, and one for devices designed for "Constrained-
   Node Networks" [RFC7228].  An SRP server is most likely an
   authoritative DNS server, or else is updating an authoritative DNS
   server.  There is no requirement that the server that is receiving
   SRP requests be the same server that is answering queries that return
   records that have been registered.

2.1.  Protocol Variants

2.1.1.  Full-featured Hosts

   Full-featured hosts are either configured manually with a
   registration domain, or use the "dr._dns-sd._udp.<domain>" query
   ([RFC6763], Section 11) to learn the default registration domain from
   the network.  RFC6763 says to discover the registration domain using
   either ".local" or a network-supplied domain name for <domain>.
   Services using SRP MUST use the domain name received through the
   DHCPv4 Domain Name option ([RFC2132], Section 3.17), if available, or
   the Neighbor Discovery DNS Search List option [RFC8106].  If the DNS
   Search List option contains more than one domain name, it MUST NOT be
   used.  If neither option is available, the Service Registration
   protocol is not available on the local network.

   Manual configuration of the registration domain can be done either by
   querying the list of available registration zones ("r._dns-sd._udp")
   and allowing the user to select one from the UI, or by any other
   means appropriate to the particular use case being addressed.  Full-
   featured devices construct the names of the SRV, TXT, and PTR records
   describing their service(s) as subdomains of the chosen service
   registration domain.  For these names they then discover the zone
   apex of the closest enclosing DNS zone using SOA queries [RFC8765].
   Having discovered the enclosing DNS zone, they query for the
   "_dnssd-srp._tcp.<zone>" SRV record to discover the server to which
   they should send DNS updates.  Hosts that support SRP updates using
   TLS use the "_dnssd-srp-tls._tcp.<zone>" SRV record instead.

2.1.2.  Constrained Hosts

   For devices designed for Constrained-Node Networks [RFC7228] some
   simplifications are available.  Instead of being configured with (or
   discovering) the service registration domain, the (proposed) special-
   use domain name (see [RFC6761]) "default.service.arpa" is used.  The
   details of how SRP server(s) are discovered will be specific to the
   constrained network, and therefore we do not suggest a specific
   mechanism here.

   SRP clients on constrained networks are expected to receive from the
   network a list of SRP servers with which to register.  It is the
   responsibility of a Constrained-Node Network supporting SRP to
   provide one or more SRP server addresses.  It is the responsibility
   of the SRP server supporting a Constrained-Node Network to handle the
   updates appropriately.  In some network environments, updates may be
   accepted directly into a local "default.service.arpa" zone, which has
   only local visibility.  In other network environments, updates for
   names ending in "default.service.arpa" may be rewritten internally to
   names with broader visibility.

2.1.3.  Why two variants?

   The reason for these different assumptions is that low-power devices
   that typically use Constrained-Node Networks may have very limited
   battery power.  The series of DNS lookups required to discover an SRP
   server and then communicate with it will increase the power required
   to advertise a service; for low-power devices, the additional
   flexibility this provides does not justify the additional use of
   power.  It is also fairly typical of such networks that some network
   service information is obtained as part of the process of joining the
   network, and so this can be relied upon to provide nodes with the
   information they need.

   Networks that are not constrained networks can more complicated
   topologies at the Internet layer.  Nodes connected to such networks
   can be assumed to be able to do DNSSD service registration domain
   discovery.  Such networks are generally able to provide registration
   domain discovery and routing.  By requiring the use of TCP, the
   possibility of off-network spoofing is eliminated.

2.2.  Protocol Details

   We will discuss several parts to this process: how to know what to
   publish, how to know where to publish it (under what name), how to
   publish it, how to secure its publication, and how to maintain the
   information once published.

2.2.1.  What to publish

   We refer to the DNS Update message sent by services using SRP as an
   SRP update.  Three types of updates appear in an SRP update: Service
   Discovery records, Service Description records, and Host Description
   records.

   *  Service Discovery records are one or more PTR RRs, mapping from
      the generic service type (or subtype) to the specific Service
      Instance Name.
   *  Service Description records are exactly one SRV RR, exactly one
      KEY RR, and one or more TXT RRs, all with the same name, the
      Service Instance Name ([RFC6763], Section 4.1).  In principle
      Service Description records can include other record types, with
      the same Service Instance Name, though in practice they rarely do.
      The Service Instance Name MUST be referenced by one or more
      Service Discovery PTR records, unless it is a placeholder service
      registration for an intentionally non-discoverable service name.
   *  The Host Description records for a service are a KEY RR, used to
      claim exclusive ownership of the service registration, and one or
      more RRs of type A or AAAA, giving the IPv4 or IPv6 address(es) of
      the host where the service resides.

   RFC 6763 describes the details of what each of these types of updates
   contains and is the definitive source for information about what to
   publish; the reason for summarizing this here is to provide the
   reader with enough information about what will be published that the
   service registration process can be understood at a high level
   without first learning the full details of DNS-SD.  Also, the
   "Service Instance Name" is an important aspect of first-come, first-
   serve naming, which we describe later on in this document.

2.2.2.  Where to publish it

   Multicast DNS uses a single namespace, ".local", which is valid on
   the local link.  This convenience is not available for DNS-SD using
   the DNS protocol: services must exist in some specific unicast
   namespace.

   As described above, full-featured devices are responsible for knowing
   in what domain they should register their services.  Devices made for
   Constrained-Node Networks register in the (proposed) special use
   domain name [RFC6761] "default.service.arpa", and let the SRP server
   handle rewriting that to a different domain if necessary.

2.2.3.  How to publish it

   It is possible to issue a DNS Update that does several things at
   once; this means that it's possible to do all the work of adding a
   PTR resource record to the PTR RRset on the Service Name, and
   creating or updating the Service Instance Name and Host Description,
   in a single transaction.

   An SRP update takes advantage of this: it is implemented as a single
   DNS Update message that contains a service's Service Discovery
   records, Service Description records, and Host Description records.

   Updates done according to this specification are somewhat different
   than regular DNS Updates as defined in RFC2136.  The RFC2136 update
   process can involve many update attempts: you might first attempt to
   add a name if it doesn't exist; if that fails, then in a second
   message you might update the name if it does exist but matches
   certain preconditions.  Because the registration protocol uses a
   single transaction, some of this adaptability is lost.

   In order to allow updates to happen in a single transaction, SRP
   updates do not include update prerequisites.  The requirements
   specified in Section 2.3 are implicit in the processing of SRP
   updates, and so there is no need for the service sending the SRP
   update to put in any explicit prerequisites.

2.2.3.1.  How DNS-SD Service Registration differs from standard RFC2136
          DNS Update

   DNS-SD Service Registration is based on standard RFC2136 DNS Update,
   with some differences:

   *  It implements first-come first-served name allocation, protected
      using SIG(0) [RFC2931].
   *  It enforces policy about what updates are allowed.
   *  It optionally performs rewriting of "default.service.arpa" to some
      other domain.
   *  It optionally performs automatic population of the address-to-name
      reverse mapping domains.
   *  An SRP server is not required to implement general DNS Update
      prerequisite processing.
   *  SRP clients are allowed to send updates to the generic domain
      "default.service.arpa"

2.2.4.  How to secure it

   Traditional DNS update is secured using the TSIG protocol, which uses
   a secret key shared between the DNS Update client (which issues the
   update) and the server (which authenticates it).  This model does not
   work for automatic service registration.

   The goal of securing the DNS-SD Registration Protocol is to provide
   the best possible security given the constraint that service
   registration has to be automatic.  It is possible to layer more
   operational security on top of what we describe here, but what we
   describe here is an improvement over the security of mDNS.  The goal
   is not to provide the level of security of a network managed by a
   skilled operator.

2.2.4.1.  First-Come First-Served Naming

   First-Come First-Serve naming provides a limited degree of security:
   a service that registers its service using DNS-SD Registration
   protocol is given ownership of a name for an extended period of time
   based on the key used to authenticate the DNS Update.  As long as the
   registration service remembers the name and the key used to register
   that name, no other service can add or update the information
   associated with that.  FCFS naming is used to protect both the
   Service Description and the Host Description.

2.2.5.  Service Behavior

2.2.5.1.  Public/Private key pair generation and storage

   The service generates a public/private key pair.  This key pair MUST
   be stored in stable storage; if there is no writable stable storage
   on the SRP client, the SRP client MUST be pre-configured with a
   public/private key pair in read-only storage that can be used.  This
   key pair MUST be unique to the device.  This key pair MUST be unique
   to the device.  A device with rewritable storage should retain this
   key indefinitely.  When the device changes ownership, it may be
   appropriate to erase the old key and install a new one.  Therefore  Therefore,
   the key MAY be overwritten as SRP client on the device SHOULD provide a mechanism to overwrite
   the key, for example as the result of a full reset of the device
   (e.g., a "factory reset"). reset."

   When sending DNS updates, the service includes a KEY record
   containing the public portion of the key in each Host Description
   update and each Service Description update.  Each KEY record MUST
   contain the same public key.  The update is signed using SIG(0),
   using the private key that corresponds to the public key in the KEY
   record.  The lifetimes of the records in the update is set using the
   EDNS(0) Update Lease option [I-D.sekar-dns-ul].

   The KEY record in Service Description updates MAY be omitted for
   brevity; if it is omitted, the SRP server MUST behave as if the same
   KEY record that is given for the Host Description is also given for
   each Service Description for which no KEY record is provided.
   Omitted KEY records are not used when computing the SIG(0) signature.

2.2.5.2.  Name Conflict Handling

   Both Host Description records and Service Description Records can
   have names that result in name conflicts.  Service Discovery records
   cannot have name conflicts.  If any Host Description or Service
   Description record is found by the server to have a conflict with an
   existing name, the server will respond to the SRP Update with a
   YXDOMAIN rcode.  In this case, the Service MUST either abandon the
   service registration attempt, or else choose a new name.

   There is no specific requirement for how this is done; typically,
   however, the service will append a number to the preferred name.
   This number could be sequentially increasing, or could be chosen
   randomly.  One existing implementation attempts several sequential
   numbers before choosing randomly.  So for instance, it might try
   host.service.arpa, then host-1.service.arpa, then host-
   2.service.arpa, then host-31773.service.arpa.

2.2.5.3.  Record Lifetimes

   The lifetime of the DNS-SD PTR, SRV, A, AAAA and TXT records
   [RFC6763] uses the LEASE field of the Update Lease option, and is
   typically set to two hours.  This means that if a device is
   disconnected from the network, it does not appear in the user
   interfaces of devices looking for services of that type for too long.

   The lifetime of the KEY records is set using the KEY-LEASE field of
   the Update Lease Option, and should be set to a much longer time,
   typically 14 days.  The result of this is that even though a device
   may be temporarily unplugged, disappearing from the network for a few
   days, it makes a claim on its name that lasts much longer.

   This means that even if a device is unplugged from the network for a
   few days, and its services are not available for that time, no other
   device can come along and claim its name the moment it disappears
   from the network.  In the event that a device is unplugged from the
   network and permanently discarded, then its name is eventually
   cleaned up and made available for re-use.

2.2.5.4.  Compression in SRV records

   Although [RFC2782] requires that the target name in the SRV record
   not be compressed, an SRP client SHOULD compress the target in the
   SRV record.  The motivation for _not_ compressing in RFC2782 is not
   stated, but is assumed to be because a caching resolver that does not
   understand the format of the SRV record might store it as binary data
   and thus return an invalid pointer in response to a query.  This does
   not apply in the case of SRP case: SRP: an SRP server needs to understand SRV
   records in order to validate the SRP update.  Compression of the
   target potentially saves substantial space in the SRP update.

2.2.5.5.  Removing published services

2.2.5.5.1.  Removing all published services

   To remove all the services registered to a service registration, particular host, the SRP
   client retransmits its most recent update with an Update Lease option
   that has a LEASE value of zero.  If the registration is to be
   permanently removed, KEY-LEASE should also be zero.  Otherwise, it
   should have the same value it had previously; this holds the name in
   reserve for when the SRP client is once again able to provide the
   service.

   SRP clients are normally expected to remove all service instances
   when removing a host.  However, in some cases a SRP client may not
   have retained sufficient state to know that some service instance is
   pointing to a host that it is removing.  An SRP server can assume
   that all service instances pointing  This method of removing
   services is intended for the case where the client is going offline
   and does not want its services advertised.  Therefore, it is
   sufficient for the client to a host entry that's send the Host Description Instruction
   (Section 2.3.1.3).

   To support this, when removing services based on the lease time being
   removed are no longer valid.  Therefore,
   zero, an SRP servers MAY server MUST remove all service instances pointing to a
   host when a host is removed, even if the SRP client doesn't remove list them
   explicitly.

2.3.  SRP Server Behavior

2.3.1.  Validation of Adds

   The  If the key lease time is nonzero, the SRP server first validates that MUST
   NOT delete the DNS Update is KEY records for these SRP clients.

2.2.5.5.2.  Removing some published services

   In some use cases a syntactically
   and semantically valid DNS Update according client may need to the rules specified remove some specific service,
   without removing its other services.  This can be accomplished in
   RFC2136.

   SRP Updates consist one
   of two ways.  The first alternative is to send a set of _instructions_ that together add one
   or more services.  Each valid SRP update
   where the only Service Discovery instruction consists either of is a remove-only
   instruction, and the only Service Description instruction is a
   remove-only instruction.  In this case, the host lease will be
   updated with the lease time provided in the SRP update.

   The second alternative is to send a normal SRP update, but as in the
   first alternative, including a Service Discovery Instruction and a
   Service Description that delete the service being removed.  This can
   be particularly useful when one service is being replaced with
   another, since this can be done in a single SRP Update.

   In neither of these cases is it permissible to delete the host.  All
   services must point to a host.  If a host is to be deleted, this must
   be done using the zero-lease deletion method.

2.3.  SRP Server Behavior

2.3.1.  Validation of Adds

   The SRP server first validates that the DNS Update is a syntactically
   and semantically valid DNS Update according to the rules specified in
   RFC2136.

   SRP Updates consist of a set of _instructions_ that together add or
   remove one or more services.  Each instruction consists either of a
   single add, a single delete, or a delete optionally followed by an
   add.  When an instruction contains a delete and an add, the delete
   MUST precede the add.

   The SRP server checks each instruction in the SRP update to see that
   it is either a Service Discovery update, a Service Description
   update, or a Host Description update.  Order matters in DNS updates.
   Specifically, deletes must precede adds for records that the deletes
   would affect; otherwise the add will have no effect.  This is the
   only ordering constraint; aside from this constraint, updates may
   appear in whatever order is convenient when constructing the update.

   Because the SRP update is a DNS update, it MUST contain a single
   question that indicates the zone to be updated.  Every delete and
   update in an SRP update MUST be within the zone that is specified for
   the SRP Update.

2.3.1.1.  Service Discovery Instruction

   An Instruction instruction is a Service Discovery Instruction if it contains

   *  exactly one "Add to an RRSet" or one "Delete an RR from an RRSet"
      ([RFC2136], Section 2.5.1) RR, RR update,
   *  which is updates a PTR RR,
   *  which points to a Service Instance Name
   *  for which a Service Description Instruction is present in the SRP
      Update.
      Update

   *  if the Service Discovery Instructions do update is an "Add to an RRSet"
      instruction, the Service Description Instruction does not match if
      it does not contain any deletes, and an "Add to an RRset" update for the SRV RR
      describing that service.
   *  if the Service Discovery Instruction is an "Delete an RR from an
      RRSet" update, the Service Description Instruction does not match
      if it contains an "Add to an RRset" update.
   *  Service Discovery Instructions do not contain any other adds. add or
      delete updates.

2.3.1.2.  Service Description Instruction

   An Instruction instruction is a Service Description Instruction if, for the
   appropriate Service Instance Name, it contains

   *  exactly one "Delete all RRsets from a name" update for the service
      instance name ([RFC2136], Section 2.5.3),
   *  exactly  zero or one "Add to an RRset" SRV RR,
   *  zero or one "Add to an RRset" KEY RR that that, if present, contains
      the public key corresponding to the private key that was used to
      sign the message (if present, the KEY MUST match the KEY RR given
      in the Host Description),
   *  one  zero or more "Add to an RRset" TXT RRs,
   *  and  If there is one "Add to an RRset" SRV update, there MUST be at
      least one "Add to an RRset" TXT update.
   *  the target of the SRV RR Add Add, if present points to a hostname for
      which there is a Host Description Instruction in the SRP Update. Update,
      or
   *  if there is no "Add to an RRset" SRV RR, then there is an existing
      SRV RR on the name specified in the "Delete all RRsets from a
      name" update that to a hostname for which there is a Host
      Description Instruction in the SRP Update, and there is a KEY RR
      on that name which matches the key with which the SRP Update was
      signed.
   *  Service Descriptions Instructions do not modify any other RRs.

   An SRP server MUST correctly handle compressed names in the SRV
   target.

2.3.1.3.  Host Description Instruction

   An Instruction instruction is a Host Description Instruction if, for the
   appropriate hostname, it contains

   *  exactly one "Delete all RRsets from a name" RR,
   *  one or more "Add to an RRset" RRs of type A and/or AAAA,
   *  A and/or AAAA records must be of sufficient scope to be reachable
      by all hosts that might query the DNS.  If a link-scope address or
      IPv4 autoconfiguration address is provided by the SRP client, the
      SRP server MUST treat this as if no address records were received;
      that is, the Host Description is not valid.
   *  exactly one "Add to an RRset" RR that adds a KEY RR that contains
      the public key corresponding to the private key that was used to
      sign the message,
   *  there is a Service Instance Name Instruction in the SRP update for
      which the SRV RR that is added points to the hostname being
      updated by this update.
   *  Host Description updates do not modify any other records.

2.3.2.  Valid SRP Update Requirements

   An SRP Update MUST include at zero or more Service Discovery
   Instructions, the same number of Service Description Instructions,
   and exactly one Host Description Instruction.  A DNS Update that does
   not is not an SRP update.  A DNS Update that contains any other adds,
   any other deletes, or any prerequisites, is not an SRP update.  Such
   messages should either be processed as regular RFC2136 updates,
   including access control checks and constraint checks, if supported,
   or else rejected with RCODE=REFUSED.

   In addition, in order for an update to be a valid SRP update, the
   target of every Service Discovery Instruction MUST be a Service
   Description Instruction that is present in the SRP Update.  There
   MUST NOT be any Service Description Instruction to which no Service
   Discovery Instruction points.  The target of the SRV record in every
   Service Description instruction MUST be the single Host Description
   Instruction.

   If the definitions of each of these instructions are followed
   carefully and the update requirements are validated correctly, many
   DNS Updates that look very much like SRP updates nevertheless will
   fail to validate.  For example, a DNS update that contains an RRset
   Add to a Service Name and an RRset Add to a Service Instance Name,
   where the Service Name does not reference the Service Instance Name,
   is not a valid SRP update message, but may be a valid RFC2136 update.

2.3.3.  FCFS Name And Signature Validation

   Assuming that a DNS Update message has been validated with these
   conditions and is a valid SRP Update, the server checks that the name
   in the Host Description Instruction exists.  If so, then the server
   checks to see if the KEY record on that name is the same as the KEY
   record in the Host Description Instruction.  The server performs the
   same check for the KEY records in any Service Description
   Instructions.  For KEY records that were omitted from Service
   Description Instructions, the KEY from the Host Description
   Instruction is used.  If any existing KEY record corresponding to a
   KEY record in the SRP Update does not match the KEY same record in
   the SRP Update (whether provided or taken from the Host Description
   Instruction), then the server MUST reject the SRP Update with the
   YXDOMAIN RCODE.

   Otherwise, the server validates the SRP Update using SIG(0) on the
   public key in the KEY record of the Host Description update.  If the
   validation fails, the server MUST reject the SRP Update with the
   REFUSED RCODE.  Otherwise, the SRP update is considered valid and
   authentic, and is processed according to the method described in
   RFC2136.

   KEY record updates omitted from Service Description update are
   processed as if they had been explicitly present: every Service
   Description that is updated MUST, after the update, have a KEY RR,
   and it must be the same KEY RR that is present in the Host
   Description to which the Service Description refers.

2.3.4.  SRP Update response

   The status that is returned depends on the result of processing the
   update, and can be either SUCCESS or SERVFAIL: all other possible
   outcomes should already have been accounted for when applying the
   constraints that qualify the update as an SRP Update.

2.3.5.  Optional Behavior

   The server MAY add a Reverse Mapping that corresponds to the Host
   Description.  This is not required because the Reverse Mapping serves
   no protocol function, but it may be useful for debugging, e.g. in
   annotating network packet traces or logs.  In order for the server to
   add a reverse mapping update, it must be authoritative for the zone
   or have credentials to do the update.  The SRP client MAY also do a
   reverse mapping update if it has credentials to do so.

   The server MAY apply additional criteria when accepting updates.  In
   some networks, it may be possible to do out-of-band registration of
   keys, and only accept updates from pre-registered keys.  In this
   case, an update for a key that has not been registered should be
   rejected with the REFUSED RCODE.

   There are at least two benefits to doing this rather than simply
   using normal SIG(0) DNS updates.  First, the same registration
   protocol can be used in both cases, so both use cases can be
   addressed by the same service implementation.  Second, the
   registration protocol includes maintenance functionality not present
   with normal DNS updates.

   Note that the semantics of using SRP in this way are different than
   for typical RFC2136 implementations: the KEY used to sign the SRP
   update only allows the SRP client to update records that refer to its
   Host Description.  RFC2136 implementations do not normally provide a
   way to enforce a constraint of this type.

   The server may also have a dictionary of names or name patterns that
   are not permitted.  If such a list is used, updates for Service
   Instance Names that match entries in the dictionary are rejected with
   YXDOMAIN.

3.  TTL Consistency

   All RRs within an RRset are required to have the same TTL
   (Clarifications to the DNS Specification [RFC2181], Section 5.2).  In
   order to avoid inconsistencies, SRP places restrictions on TTLs sent
   by services and requires that SRP servers enforce consistency.

   Services sending SRP updates MUST use consistent TTLs in all RRs
   within the SRP update.

   SRP update servers MUST check that the TTLs for all RRs within the
   SRP update are the same.  If they are not, the SRP update MUST be
   rejected with a REFUSED RCODE.

   Additionally, when adding RRs to an RRset, for example when
   processing Service Discovery records, the server MUST use the same
   TTL on all RRs in the RRset.  How this consistency is enforced is up
   to the implementation.

   TTLs sent in SRP updates are advisory: they indicate the SRP client's
   guess as to what a good TTL would be.  SRP servers may override these
   TTLs.  SRP servers SHOULD ensure that TTLs are reasonable: neither
   too long nor too short.  The TTL should never be longer than the
   lease time (Section 4.1).  Shorter TTLs will result in more frequent
   data refreshes; this increases latency on the DNS-SD client side,
   increases load on any caching resolvers and on the authoritative
   server, and also increases network load, which may be an issue for
   constrained networks.  Longer TTLs will increase the likelihood that
   data in caches will be stale.  TTL minimums and maximums SHOULD be
   configurable by the operator of the SRP server.

4.  Maintenance

4.1.  Cleaning up stale data

   Because the DNS-SD registration protocol is automatic, and not
   managed by humans, some additional bookkeeping is required.  When an
   update is constructed by the SRP client, it MUST include an EDNS(0)
   Update Lease Option [I-D.sekar-dns-ul].  The Update Lease Option
   contains two lease times: the Lease Time and the Key Lease Time.

   These leases are promises, similar to DHCP leases [RFC2131], from the
   SRP client that it will send a new update for the service
   registration before the lease time expires.  The Lease time is chosen
   to represent the time after the update during which the registered
   records other than the KEY record should be assumed to be valid.  The
   Key Lease time represents the time after the update during which the
   KEY record should be assumed to be valid.

   The reasoning behind the different lease times is discussed in the
   section on first-come, first-served naming (Section 2.2.4.1).  SRP
   servers may be configured with limits for these values.  A default
   limit of two hours for the Lease and 14 days for the SIG(0) KEY are
   currently thought to be good choices.  Constrained devices with
   limited battery that wake infrequently are likely to negotiate longer
   leases.  SRP clients that are going to continue to use names on which
   they hold leases should update well before the lease ends, in case
   the registration service is unavailable or under heavy load.

   The lease time applies specifically to the host.  All service
   instances, and all service entries for such service instances, depend
   on the host.  When the lease on a host expires, the host and all
   services that reference it MUST be removed at the same time-it is
   never valid for a service instance to remain when the host it
   references has been removed.  If the KEY record for the host is to
   remain, the KEY record for any services that reference it MUST also
   remain.  However, the service PTR record MUST be removed, since it
   has no key associated with it, and since it is never valid to have a
   service PTR record for which there is no service instance on the
   target of the PTR record.

   SRP Servers SHOULD also track a lease time per service instance.  The
   reason for doing this is that a client may re-register a host with a
   different set of services, and not remember that some different
   service instance had previously been registered.  In this case, when
   that service instance lease expires, the SRP server SHOULD remove the
   service instance (although the KEY record for the service instance
   SHOULD be retained until the key lease on that service expires).
   This is beneficial because if the SRP client continues to renew the
   host, but never mentions the stale service again, the stale service
   will continue to be advertised.

   The SRP server MUST include an EDNS(0) Update Lease option in the
   response if the lease time proposed by the service has been shortened
   or lengthened.  The service MUST check for the EDNS(0) Update Lease
   option in the response and MUST use the lease times from that option
   in place of the options that it sent to the server when deciding when
   to update its registration.  The times may be shorter or longer than
   those specified in the SRP update; the SRP client must honor them in
   either case.

   SRP clients should assume that each lease ends N seconds after the
   update was first transmitted, where N is the lease duration.  Servers
   should assume that each lease ends N seconds after the update that
   was successfully processed was received.  Because the server will
   always receive the update after the SRP client sent it, this avoids
   the possibility of misunderstandings.

   SRP servers MUST reject updates that do not include an EDNS(0) Update
   Lease option.  Dual-use servers MAY accept updates that don't include
   leases, but SHOULD differentiate between SRP updates and other
   updates, and MUST reject updates that would otherwise be SRP updates
   if they do not include leases.

   Lease times have a completely different function than TTLs.  On an
   authoritative DNS server, the TTL on a resource record is a constant:
   whenever that RR is served in a DNS response, the TTL value sent in
   the answer is the same.  The lease time is never sent as a TTL; its
   sole purpose is to determine when the authoritative DNS server will
   delete stale records.  It is not an error to send a DNS response with
   a TTL of 'n' when the remaining time on the lease is less than 'n'.

5.  Sleep Proxy

   Another use of SRP is for devices that sleep to reduce power
   consumption.

   In this case, in addition to the DNS Update Lease option
   [I-D.sekar-dns-ul] described above, the device includes an EDNS(0)
   OWNER Option [I-D.cheshire-edns0-owner-option].

   The EDNS(0) Update Lease option constitutes a promise by the device
   that it will wake up before this time elapses, to renew its
   registration and thereby demonstrate that it is still attached to the
   network.  If it fails to renew the registration by this time, that
   indicates that it is no longer attached to the network, and its
   registration (except for the KEY in the Host Description) should be
   deleted.

   The EDNS(0) OWNER Option indicates that the device will be asleep,
   and will not be receptive to normal network traffic.  When a DNS
   server receives a DNS Update with an EDNS(0) OWNER Option, that
   signifies that the SRP server should set up a proxy for any IPv4 or
   IPv6 address records in the DNS Update message.  This proxy should
   send ARP or ND messages claiming ownership of the IPv4 and/or IPv6
   addresses in the records in question.  In addition, the proxy should
   answer future ARP or ND requests for those IPv4 and/or IPv6
   addresses, claiming ownership of them.  When the DNS server receives
   a TCP SYN or UDP packet addressed to one of the IPv4 or IPv6
   addresses for which it proxying, it should then wake up the sleeping
   device using the information in the EDNS(0) OWNER Option.  At present
   version 0 of the OWNER Option specifies the "Wake-on-LAN Magic
   Packet" that needs to be sent; future versions could be extended to
   specify other wakeup mechanisms.

   Note that although the authoritative DNS server that implements the
   SRP function need not be on the same link as the sleeping host, the
   Sleep Proxy must be on the same link.

   It is not required that sleepy nodes on a Constrained-Node Network
   support sleep proxy.  Such devices may have different mechanisms for
   dealing with sleep and wakeup.  An SRP registration for such a device
   will be useful regardless of the mechanism whereby messages are
   delivered to the sleepy end device.  For example, the message might
   be held in a buffer for an extended period of time by an intermediate
   device on a mesh network, and then delivered to the device when it
   wakes up.  The exact details of such behaviors are out of scope for
   this document.

6.  Security Considerations
6.1.  Source Validation

   SRP updates have no authorization semantics other than first-come,
   first-served.  This means that if an attacker from outside of the
   administrative domain of the server knows the server's IP address, it
   can in principle send updates to the server that will be processed
   successfully.  Servers should therefore be configured to reject
   updates from source addresses outside of the administrative domain of
   the server.

   For updates sent to an anycast IP address of an SRP server, this
   validation must be enforced by every router on the path from the
   Constrained-Device Network to the unconstrained portion of the
   network.  For TCP updates, the initial SYN-SYN+ACK handshake prevents
   updates being forged by an off-network attacker.  In order to ensure
   that this handshake happens, Service Discovery Protocol servers
   relying on three-way-handshake validation MUST NOT accept TCP Fast
   Open payloads.  If the network infrastructure allows it, an SRP
   server MAY accept TCP Fast Open payloads if all such packets are
   validated along the path, and the network is able to reject this type
   of spoofing at all ingress points.

   Note that these rules only apply to the validation of SRP updates.  A
   server that accepts updates from SRP clients may also accept other
   DNS updates, and those DNS updates may be validated using different
   rules.  However, in the case of a DNS service that accepts SRP
   updates, the intersection of the SRP update rules and whatever other
   update rules are present must be considered very carefully.

   For example, a normal, authenticated DNS update to any RR that was
   added using SRP, but that is authenticated using a different key,
   could be used to override a promise made by the registration
   protocol, by replacing all or part of the service registration
   information with information provided by an SRP client.  An
   implementation that allows both kinds of updates should not allow DNS
   Update clients to updateupdate records added by SRP clients that are using different authentication and
   authorization credentials. credentialsto to update records added by SRP clients.

6.2.  SRP Server Authentication

   This specification does not provide a mechanism for validating
   responses from DNS servers to SRP clients.  In the case of
   Constrained Network/Constrained Node clients, such validation isn't
   practical because there's no way to establish trust.  In principle, a
   KEY RR could be used by a non-constrained SRP client to validate
   responses from the server, but this is not required, nor do we
   specify a mechanism for determining which key to use.

6.3.  Required Signature Algorithm

   For validation, SRP servers MUST implement the ECDSAP256SHA256
   signature algorithm.  SRP servers SHOULD implement the algorithms
   specified in [RFC8624], Section 3.1, in the validation column of the
   table, that are numbered 13 or higher and have a "MUST",
   "RECOMMENDED", or "MAY" designation in the validation column of the
   table.  SRP clients MUST NOT assume that any algorithm numbered lower
   than 13 is available for use in validating SIG(0) signatures.

7.  Privacy Considerations

   Because DNSSD SRP updates can be sent off-link, the privacy
   implications of SRP are different than for multicast DNS responses.
   Host implementations that are using TCP SHOULD also use TLS if
   available.  Server implementations MUST offer TLS support.  The use
   of TLS with DNS is described in [RFC7858] and [RFC8310].

   Hosts that implement TLS support SHOULD NOT fall back to TCP; since
   servers are required to support TLS, it is entirely up to the host
   implementation whether to use it.

   Public keys can be used as identifiers to track hosts.  SRP servers
   MAY elect not to return KEY records for queries for SRP
   registrations.

8.  Delegation of 'service.arpa.'

   In order to be fully functional, the owner of the 'arpa.' zone must
   add a delegation of 'service.arpa.' in the '.arpa.' zone [RFC3172].
   This delegation should be set up as was done for 'home.arpa', as a
   result of the specification in Section 7 of [RFC8375].

9.  IANA Considerations

9.1.  Registration and Delegation of 'service.arpa' as a Special-Use
      Domain Name

   IANA is requested to record the domain name 'service.arpa.' in the
   Special-Use Domain Names registry [SUDN].  IANA is requested, with
   the approval of IAB, to implement the delegation requested in
   Section 8.

   IANA is further requested to add a new entry to the "Transport-
   Independent Locally-Served Zones" subregistry of the the "Locally-
   Served DNS Zones" registry [LSDZ].  The entry will be for the domain
   'service.arpa.' with the description "DNS-SD Registration Protocol
   Special-Use Domain", listing this document as the reference.

9.2.  'dnssd-srp' Service Name

   IANA is also requested to add a new entry to the Service Names and
   Port Numbers registry for dnssd-srp with a transport type of tcp.  No
   port number is to be assigned.  The reference should be to this
   document, and the Assignee and Contact information should reference
   the authors of this document.  The Description should be as follows:

   Availability of DNS Service Discovery Service Registration Protocol
   Service for a given domain is advertised using the
   "_dnssd-srp._tcp.<domain>."  SRV record gives the target host and
   port where DNSSD Service Registration Service is provided for the
   named domain.

9.3.  'dnssd-srp-tls' Service Name

   IANA is also requested to add a new entry to the Service Names and
   Port Numbers registry for dnssd-srp with a transport type of tcp.  No
   port number is to be assigned.  The reference should be to this
   document, and the Assignee and Contact information should reference
   the authors of this document.  The Description should be as follows:

   Availability of DNS Service Discovery Service Registration Protocol
   Service for a given domain over TLS is advertised using the
   "_dnssd-srp-tls._tcp.<domain>."  SRV record gives the target host and
   port where DNSSD Service Registration Service is provided for the
   named domain.

9.4.  Anycast Address

   IANA is requested to allocate an IPv6 Anycast address from the IPv6
   Special-Purpose Address Registry, similar to the Port Control
   Protocol anycast address, 2001:1::1.  The value TBD should be
   replaced with the actual allocation in the table that follows.  The
   values for the registry are:

          +----------------------+-----------------------------+
          | Attribute            | value                       |
          +----------------------+-----------------------------+
          | Address Block        | 2001:1::TBD/128             |
          +----------------------+-----------------------------+
          | Name                 | DNS-SD Service Registration |
          |                      | Protocol Anycast Address    |
          +----------------------+-----------------------------+
          | RFC                  | [this document]             |
          +----------------------+-----------------------------+
          | Allocation Date      | [date of allocation]        |
          +----------------------+-----------------------------+
          | Termination Date     | N/A                         |
          +----------------------+-----------------------------+
          | Source               | True                        |
          +----------------------+-----------------------------+
          | Destination          | True                        |
          +----------------------+-----------------------------+
          | Forwardable          | True                        |
          +----------------------+-----------------------------+
          | Global               | True                        |
          +----------------------+-----------------------------+
          | Reserved-by-protocol | False                       |
          +----------------------+-----------------------------+

                                 Table 1

10.  Acknowledgments

   Thanks to Toke H&#248;iland-J&#248;rgensen, Høiland-Jørgensen, Jonathan Hui and Esko Dijk for
   their thorough technical reviews, to Tamara Kemper for doing a nice
   developmental edit, Tim Wattenberg for doing a service implementation
   at the Montreal Hackathon at IETF 102, and Tom Pusateri for reviewing
   during the hackathon and afterwards.

11.  Normative References

   [I-D.sekar-dns-ul]
              Cheshire, S. and T. Lemon, "Dynamic DNS Update Leases",
              Work in Progress, Internet-Draft, draft-sekar-dns-ul-02, 2
              August 2018,
              <https://tools.ietf.org/html/draft-sekar-dns-ul-02>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <https://www.rfc-editor.org/info/rfc2931>.

   [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational
              Requirements for the Address and Routing Parameter Area
              Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
              September 2001, <https://www.rfc-editor.org/info/rfc3172>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 8106, DOI 10.17487/RFC8106, March 2017,
              <https://www.rfc-editor.org/info/rfc8106>.

   [RFC8375]  Pfister, P. and T. Lemon, "Special-Use Domain
              'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
              <https://www.rfc-editor.org/info/rfc8375>.

   [RFC8624]  Wouters, P. and O. Sury, "Algorithm Implementation
              Requirements and Usage Guidance for DNSSEC", RFC 8624,
              DOI 10.17487/RFC8624, June 2019,
              <https://www.rfc-editor.org/info/rfc8624>.

   [RFC8765]  Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              RFC 8765, DOI 10.17487/RFC8765, June 2020,
              <https://www.rfc-editor.org/info/rfc8765>.

   [SUDN]     "Special-Use Domain Names Registry", July 2012,
              <https://www.iana.org/assignments/special-use-domain-
              names/special-use-domain-names.xhtml>.

   [LSDZ]     "Locally-Served DNS Zones Registry", July 2011,
              <https://www.iana.org/assignments/locally-served-dns-
              zones/locally-served-dns-zones.xhtml>.

12.  Informative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [RFC6760]  Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace the AppleTalk Name Binding Protocol (NBP)",
              RFC 6760, DOI 10.17487/RFC6760, February 2013,
              <https://www.rfc-editor.org/info/rfc6760>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018,
              <https://www.rfc-editor.org/info/rfc8310>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [RFC8766]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
              2020, <https://www.rfc-editor.org/info/rfc8766>.

   [I-D.cheshire-dnssd-roadmap]
              Cheshire, S., "Service Discovery Road Map", Work in
              Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03,
              23 October 2018, <https://tools.ietf.org/html/draft-
              cheshire-dnssd-roadmap-03>.

   [I-D.cheshire-edns0-owner-option]
              Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", Work
              in Progress, Internet-Draft, draft-cheshire-edns0-owner-
              option-01, 3 July 2017, <https://tools.ietf.org/html/
              draft-cheshire-edns0-owner-option-01>.

   [ZC]       Cheshire, S. and D.H. Steinberg, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. ,
              ISBN 0-596-10100-7, December 2005.

Appendix A.  Testing using standard RFC2136-compliant servers

   It may be useful to set up a DNS server for testing that does not
   implement SRP.  This can be done by configuring the server to listen
   on the anycast address, or advertising it in the
   _dnssd-srp._tcp.<zone> SRV and _dnssd-srp-tls._tcp.<zone> record.  It
   must be configured to be authoritative for "default.service.arpa",
   and to accept updates from hosts on local networks for names under
   "default.service.arpa" without authentication, since such servers
   will not have support for FCFS authentication (Section 2.2.4.1).

   A server configured in this way will be able to successfully accept
   and process SRP updates from services that send SRP updates.
   However, no prerequisites will be applied, and this means that the
   test server will accept internally inconsistent SRP updates, and will
   not stop two SRP updates, sent by different services, that claim the
   same name(s), from overwriting each other.

   Since SRP updates are signed with keys, validation of the SIG(0)
   algorithm used by the client can be done by manually installing the
   client public key on the DNS server that will be receiving the
   updates.  The key can then be used to authenticate the client, and
   can be used as a requirement for the update.  An example
   configuration for testing SRP using BIND 9 is given in Appendix C.

Appendix B.  How to allow services to update standard RFC2136-compliant
             servers

   Ordinarily SRP updates will fail when sent to an RFC 2136-compliant
   server that does not implement SRP because the zone being updated is
   "default.service.arpa", and no DNS server that is not an SRP server
   should normally be configured to be authoritative for
   "default.service.arpa".  Therefore, a service that sends an SRP
   update can tell that the receiving server does not support SRP, but
   does support RFC2136, because the RCODE will either be NOTZONE,
   NOTAUTH or REFUSED, or because there is no response to the update
   request (when using the anycast address)

   In this case a service MAY attempt to register itself using regular
   RFC2136 DNS updates.  To do so, it must discover the default
   registration zone and the DNS server designated to receive updates
   for that zone, as described earlier, using the _dns-update._udp SRV
   record.  It can then make the update using the port and host pointed
   to by the SRV record, and should use appropriate prerequisites to
   avoid overwriting competing records.  Such updates are out of scope
   for SRP, and a service that implements SRP MUST first attempt to use
   SRP to register itself, and should only attempt to use RFC2136
   backwards compatibility if that fails.  Although the owner name for
   the SRV record specifies the UDP protocol for updates, it is also
   possible to use TCP, and TCP should be required to prevent spoofing.

Appendix C.  Sample BIND9 configuration for default.service.arpa.

   zone "default.service.arpa." {
     type master;
     file "/etc/bind/master/service.db";
     allow-update { key demo.default.service.arpa.; };
   };

                 Figure 1: Zone Configuration in named.conf

 $ORIGIN .
 $TTL 57600  ; 16 hours
 default.service.arpa IN SOA          ns3.default.service.arpa.
                                      postmaster.default.service.arpa. (
                 2951053287 ; serial
                 3600       ; refresh (1 hour)
                 1800       ; retry (30 minutes)
                 604800     ; expire (1 week)
                 3600       ; minimum (1 hour)
 )
                         NS           ns3.default.service.arpa.
                         SRV 0 0 53   ns3.default.service.arpa.
 $ORIGIN default.service.arpa.
 $TTL 3600   ; 1 hour
 _ipps._tcp              PTR          demo._ipps._tcp
 $ORIGIN _ipps._tcp.default.service.arpa.
 demo                    TXT          "0"
                         SRV 0 0 9992 demo.default.service.arpa.
 $ORIGIN _udp.default.service.arpa.
 $TTL 3600   ; 1 hour
 _dns-update             PTR          ns3.default.service.arpa.
 $ORIGIN _tcp.default.service.arpa.
 _dnssd-srp              PTR          ns3.default.service.arpa.
 $ORIGIN default.service.arpa.
 $TTL 300    ; 5 minutes
 ns3                     AAAA         2001:db8:0:1::1
 $TTL 3600   ; 1 hour
 demo                    AAAA         2001:db8:0:2::1
                         KEY 513 3 13 (
                            qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
                            9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
                         ); alg = ECDSAP256SHA256 ; key id = 15008
                         AAAA    ::1

                      Figure 2: Example Zone file

Authors' Addresses

   Ted Lemon
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Email: mellon@fugue.com
   Stuart Cheshire
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Phone: +1 408 974 3207
   Email: cheshire@apple.com